This invention is directed to the threaded fastener arts and more particularly to a novel and improved method and apparatus for gaging threaded fasteners and still more particularly for gaging point threads of threaded fasteners, or other special threads.
Such gaging is preferably carried out in order to determine deviations of the dimensions of such fasteners from the specified or desired dimensions, which data may be utilized for statistical in-process control, for lot control and the like. While the method and apparatus of the invention may be utilized for measuring or gaging of other types of special threads, the ensuing discussion will be directed primarily to the gaging of point threads of poly-arcuate or lobular fasteners.
Such point threads may comprise thread-forming or thread cutting threads for forming a thread in a pilot hole during driving of the fastener, or an anticross-threading tapered lead-in portion of the fastener, for engaging a pretapped workpiece. Other forms of special threads which may be gaged in accordance with the invention include the threads in a work-entering section of a dual lobulated type of fastener such as shown for example in U.S. Pat. No. 4,040,328, or an intermediate, initial thread-forming section of a step-taper fastener of the type shown in U.S. Pat. No. 4,194,430.
The gaging of lobular fasteners has traditionally been a somewhat complicated task, because the degree of lobulation or out-of-roundness is just as important as the basic thread dimensions in the formation of these fasteners. The degree of lobulation, or out of round, is usually defined as half the diametral difference between the inscribed and circumscribed circles of a cross-section arcuate form of the fastener. Such lobular fasteners are used extensively both as thread-forming screws and for self-locking, or sealing purposes in pre-tapped holes.
In either of these general applications control of the degree of lobulation is important in assuring proper performance of the product. For example, this degree of lobulation (commonly designated K) must exceed a certain minimum for thread-forming with a reasonable driving effort or for entry into a preformed thread and adequate sealing or self-locking with acceptable driving effort. On the other hand, if this K or degree of lobulation exceeds a certain maximum amount, the tensile load-carrying ability of the joint in the work may be seriously comprised, due to the reduced surface contact area of the threads
In the case of lobular thread rolling or thread-forming screws, it is estimated that as much as 95% or more of the work or effort in forming internal threads is performed by the lead or point threads. The lead threads may be considered to be enveloped or circumscribed within a lobular, frustoconical shape with lobes corresponding in number with those of the body threads. The angle of this cone and number of lead threads is governed by the individual product specifications from one fastener to another. However, it will be appreciated that the cross-sectional dimensions will vary from thread to thread, along this tapered lead-in portion.
The driving effort in the installation of such a thread-rolling or thread-forming screw is generally indicated by the applied torque measured over the degrees of rotation (expressed in radians) of the screw. As the screw is driven, the rotation is accompanied by a progressive radial outward movement of the lead threads with increasing torque in forming complementary internal mating threads in the workpiece or nut. Consequently, the method and apparatus of the invention is used in evaluating the relationship of the K or out-of-round dimension to the rotation of the lead threads of such a thread-forming fastener, to assure the same are formed in such a manner as to obtain the desired driving effort and thread forming action.
In the past, conventional screw body threads have often been measured by threading each fastener to be tested into each of a pair of so-called ring or functional gages, which have internal threads corresponding with maximum and minimum allowable external thread dimensions of the fastener. An acceptable product was considered to be one which could be threaded into the maximum gage but not into the minimum gage. However, it should be recognized that non-entry into the minimum gage could be the result of only a single over-size thread element or portion, and would not imply any control whatever on the individual minimum thread dimensions of the fastener. In recent years this condition has been alleviated somewhat through the use of pitch diameter micrometers, individual element indicator gages and other techniques. However, these methods have not been successfully applied to the measurement of point threads.
Past methods of gaging the cross-sectional dimensions of body threads of lobular thread-forming and self-locking screws used hand micrometers. In a first measurement, a multi-anvil type micrometer was used, in which the fastener was rotated to obtain the maximum reading; that is, the diameter of the circumscribing circle "C". In a second operation, a more or less conventional micrometer was used to obtain the "D" dimension, which is a cross-section from a high point of one lobe to a low point opposite. It was not the practice, however, to calculate the difference between these two micrometer readings; i.e., the K, or out of round dimension.
That is, the dimensions C and D of the fastener alone, even if themselves within acceptable limits, do not guarantee an acceptable K or out of round dimension. Moreover, the two-step inspection process utilizing two micrometers is quite cumbersome to perform, is time-consuming, requiring individual handling and inspection of each part to be inspected and gaged in this method, and also effectively doubles the potential for measurement error. These techniques have not been applied to the measurement of point threads.